Rotors of large turbogenerators, before being built into the stator, are normally subjected at the manufacturer's to a test, referred to as “warm-up”, which follows the balancing of the rotor in the cold state in the centrifuge. The “warm-up” is intended to simulate the thermally induced unbalances of the rotor which occur during the normal operation of the ready-assembled machine, so that a deviating oscillation behavior can be detected at an early stage and corrected even during production.
A thermally induced warping of the rotor and the oscillations generated as a result of this reversible and reproducible unbalance are based on two critical mechanisms:    the rotor has reached its nominal rotational speed, and the rotor winding is held against the centrifugal forces, for example by means of wedges.    In this state, an elongation of the rotor winding occurs and causes a relative movement between the rotor body and the rotor winding.
The forces and mass displacements may, in principle, bring about a variation in the balanced state of the rotor. The measured rotor oscillation will therefore vary. While the turbogenerator is in operation, this variation occurs during an increase in load on a power station and during the reduction in power of the latter and can be tested on the solitary generator rotor in a predetermined “warm-up” test method.
A test method employed at the present time comprises the investigation of the oscillation behavior of a balanced rotor which rotates in a test stand at the nominal rotational speed, while at the same time a predetermined rotor temperature profile is applied. The rise in the mean winding temperature (MWT) is in this case generated as a result of the application of an exciting current, such as will be fed in during the operation of the machine at the installation location. The rotor winding is thereafter cooled, in that the exciting system is switched off and the rotor continues to be operated at nominal rotational speed in the ventilated test stand. This test method, admittedly, is very close to the mechanisms which give rise to thermal unbalance during subsequent operation. However, it requires a considerable outlay in terms of the set-up of the test stand: electromagnetic shields, cooling, an exciting system, sufficient drive power and suitable measuring and monitoring systems.
In another method, the rotor is first balanced in the centrifuge and is then rotated at nominal rotational speed until a predetermined mean winding temperature is reached by virtue of the ventilation losses (flow-dynamic losses which increase the temperature of the cooling medium and consequently of the copper). This method may last through a disproportionately long time during which both the rotor body and the winding heat up. As a result, some important mechanisms of the rotor unbalance are not reproduced, and experience has shown that this leads to disadvantages in terms of the evidential force of such tests.